1. Field of the Invention
The present invention relates to cleaning apparatus, and more particularly, to cleaning apparatus for use with crushing machines.
2. Description of the Prior Art
Coal, minerals and other mined, quarried or dredged organic and inorganic materials are often processed in size reduction machines, such as pulverizers, crushers or mills. Screens of a predetermined mesh are generally positioned upstream of the discharge end to permit only particles of a desired maximum size to pass. Depending upon the flow properties of the particular material, the size of the screen opening can reach a critical dimension, beyond which any further reduction in the size of the screen openings will cause the material to arch over the openings, thus blinding, or blocking the screen. Arching is caused generally by the increase in shear strength generated by interparticle friction, compaction, temperature or moisture build up. Blinding the screen leads to flow stoppage.
The problem is particularly acute when the material is exposed to weather. Coal, for example, will flow with minimal problems when dry; but, when wetted from rain, snow or process water, coal develops bonding strength sufficient to cause the coal to arch over 8 mesh screen. (2.36 millimeters, specified by the American Society of Testing Materials to achieve a maximum particle size). In addition, the natural moisture content of materials can contribute to increased shear strength and, consequently, arching.
A hammermill is the preferred material reduction machine for processing coal because it minimizes moisture loss. The natural moisture content of coal is an important parameter in assessing product quality.
In the coal industry, as well as in other organic and inorganic minerals industries, payments for material shipments in both domestic and international trade are determined by evaluating representative samples of the product. Because of the screen blinding that occurs, the samples evaluated are not truly representative, and the payments are, therefore, based on statistically incorrect representations of the contents of the entire tonnage sampled.
Several devices have been developed to clear openings in screens. Teague et al. U.S. Pat. No. 3,856,216 which issued on Dec. 24, 1974, discloses a mechanical rake oriented on the face of a bar grid for use in primary treatment of waste water. The automatic rake periodically sweeps along the entry side of the bar grids removing coarse solids which accumulate on the grid face as the waste water flows through it.
Botsch U.S. Pat. No. 4,184,957 which issued on Jan. 22, 1980, discloses a screening apparatus which includes a screen cleaner for removing coarse material from fluids. A dual screen arrangement is placed, one screen positioned behind the other, in the path of the fluid flow so that coarse materials within the fluid are retained on the face of the screens. The screen cleaner includes a motor driven cleaning carriage to which a rake is attached. The rake has a plurality of teeth which extend through the first screen and onto the face of the second screen. When the cleaning carriage is activated the rake moves upward along the screens stripping the entry sides of both screens of the particles deposited thereon.
Kreitner et al. U.S. Pat. No. 4,421,642 which issued on Dec. 20, 1983, discloses a device for clearing passages in the sieve plate of a wet comminuting machine. The sieve plate is used to separate the grinding medium within the comminuting machine from the milled suspension. The sieve plate has strips or holes through which the milled suspension flows, but which block the passage of the grinding medium. When the holes or slots in the sieve plate become blocked, a plate having sheet metal spikes or strips corresponding to the holes or slots is moved toward the sieve. As the sheet metal spikes or strips are pushed through the passages in the sieve plate, perpendicular to the holes or strips, the sieve is unclogged and further filtration can continue. The cleaning operation is repeated periodically, as required, and can be operated manually, pneumatically, hydraulically, or electrically.
Schoellhorn et al. U.S. Pat. No. 681,983, issued on Sept. 3, 1901; Shelton U.S. Pat. No. 1,670,748, issued on May 22, 1928; and Heinrich U.S. Pat. No. 3,587,983, issued on June 28, 1971, relate to cleaning mechanisms on rotary comminuting machines. The Schoellhorn patent discloses a pulverizer with a screen arrangement beneath the hammers of the comminutor. In order to prevent material from becoming lodged between the bars of the screen, the bars are caused to rock back and forth when the machine is in operation.
The Shelton Patent discloses a rotary crusher, or hammermill, which has an interior reciprocating impact plate. The reciprocating impact plate is designed to break up any crushed material which may become packed in a mass at the opening to the hammer circle. The packed mass reduces, and sometimes shuts off the feed to the machine.
The Heinrich Patent discloses a rotary crusher in which a baffle wall is periodically moved toward the crusher tools of the rotor by automatic controls to clear deposits which have collected on the baffle wall. The reciprocating and moving plates and walls described by the prior art references would not dislodge obstructions wedged in the openings of a sizing screen.
The object of the present invention is to provide a device for clearing arched material and other obstructions wedged in the openings in the screen of a material reduction machine. A further object is to provide such a clearing device which removes the obstructions from the discharge side of the screen, thus permitting the uninterrupted operation of the machine. A further object of the present invention is to provide such a parallel motion clearing device which can be easily added onto or incorporated into existing material reduction machines.